How to Repair Window Sills

A wooden sill is often the first part of a window to need repair, as the rain that falls on the window drips on to the sill and some of it may collect, resulting in flaking paint or crumbling, rotten wood. The treatment ranges from simple repainting to replacement. Since sills project from the wall, they are also prone to impact damage, which can affect concrete and stone sills.

Wet rot

This can be recognized by softening and darkening of the wood and often by severe splintering, the rotten wood falling out. Fortunately, it is fairly easy to repair; on the other hand, dry rot – recognizable by white strands on the surface and a musty smell – requires the services of a professional.

Provided the damage is not so severe that sill replacement is a better option, the first thing to do is remove all the rotten wood with a sharp chisel until you get back to sound, dry wood. Use a hot-air gun if necessary to dry out the wood.

Brush the wood with a wood hardener solution and leave this to dry. This prepares the wood for the application of exterior wood filler. Although this sets hard, it retains sufficient flexibility to be able to move with the wood as it expands and contracts with varying temperatures and humidity. The wood filler can be applied with a filling or putty knife and should be left to set. Substantial damage may need two or even three layers.

Once the filler has hardened, it can be sanded down and the whole area painted to match the surrounding wood – it is probably best to repaint the whole sill.

Some wet-rot repair systems include wood preservative sticks or tablets that you put in drilled holes in the wood surrounding the repair area to prevent future rot. The holes are concealed when the damaged area is filled.

A wet rot system cannot be used to make good dry rot, which can affect masonry as well as woodwork. In this situation, all damaged wood must be completely removed (along with any affected bricks and mortar) and the areas must be professionally sterilized before replacement materials are installed.

Repairing a concrete or stone sill

A simple crack or small hole in a concrete or stone sill can be repaired in much the same way that you deal with a crack or hole in a wall or ceiling, using an appropriate exterior filler or ready-mixed mortar.

Damage to the edge is more difficult to repair, as it involves building timber shuttering to hold the new concrete while it sets.

Chisel away all loose concrete before fitting the shuttering, using temporary timber supports. Then fill the damaged area with fresh concrete – use dry readymix – using a trowel. Once the concrete has completely set, you can remove the shuttering. A similar arrangement is used to replace a whole concrete or stone sill, but in this case, it must take the form of a box with a length of rope pinned along the bottom to provide a former for the window sill’s drip groove.